In profile turning using a cutting tool, since the direction of feed, and depth of cut, of the cutting tool changes according to the shape of the work, the chip produced by the cutting edge of the insert mounted at the tip of the cutting tool also sequentially changes in its direction of outflow, width, and thickness. For example, as shown in FIG. 29, when, after an external diameter part extending parallel to a rotation axis S is formed on an outer periphery of a work W, an attempt is made to form an end face part extending in a direction orthogonal to the axis S via a rounded part by an insert T mounted at the tip of the main body of a cutting tool (not shown in drawing), even if depth of cut to the work W surface is made constant so that the thickness of chip becomes constant, in the external diameter part narrow chip is produced in the corner cutting edge C of the insert T. In contrast to this, at the end face part the insert T is provided with feed in the direction to raise it to the outer peripheral side of the work W so that a wide chip is produced using a cutting edge of a certain length from the corner part C. Furthermore, in some cases, as the direction of outflow of the chip changes dramatically in the rounded part between these outer peripheral parts and end face parts, a wide chip may be produced even in the periphery of the corner part C depending upon the shape of the work W prior to cutting.
Therefore, even if a breaker for chip control is formed on the rake face of the insert T, in the case where the breaker is provided to produce narrow chip, when wide chip is produced in back feed cutting in corner cutting, or cutting of the rounded part, it cannot reliably be processed. Conversely, when a breaker is provided to handle wide chip, narrow chip produced during cutting of the outer periphery part is not broken and extends in a long length, winding around the shank of the cutting tool, inhibiting better chip control. Moreover, when the direction of outflow of the chip changes while cutting the rounded part between the external diameter part and the end face part, it is also difficult to reliably control such chip. However, in relation to this point, for example, Japanese Unexamined Patent Application, First Publication No. Hei 9-38807 has proposed a polygonal plate shape insert having a cutting edge, and an adjacent narrow breaker groove, on the top ridge line, and in this breaker groove a substantially hemispherical projection is formed in the vicinity of the corner part on the bisection line of the apex angle. Furthermore, an insert having in plan view a substantially elliptical projection which is long in the parallel direction to the cutting edge formed at a position equidistant from the apex angle corner parts, has been also proposed. However in such inserts, since the chip is simply flowed out along the breaker groove onto the inside of the rake face and collides with the aforementioned respective projections to be broken, the effect of fragmenting the chip and changing the direction of outflow are small. Furthermore if the width of the breaker groove between the aforementioned elliptical projection and the cutting edge is reduced to suit back feed cutting in corner cutting, the problem occurs such that chip jamming occurs while cutting the external diameter part by cutting at a large depth of cut or at high feed, or such that it is unable to control the direction of outflow of the chip by processing the rounded part.